Soda-lime-silica glass is used extensively throughout the glass manufacturing industry to fabricate a number of glass articles including glass containers (e.g., bottles, jars, etc.), products derived from flat glass (e.g., windowpanes, glass walls, doorpanes, windshields, etc.), and glass fibers, to name but a few examples. Soda-lime-silica glass is typically formed as an amorphous inorganic solid that includes 60 wt % to 80 wt % silica, 8 wt % to 18 wt % sodium oxide, 5 wt % to 15 wt % calcium oxide, and optionally 0-2 wt % aluminum oxide (Al2O3), 0-4 wt % magnesium oxide (MgO), 0-1.5 wt % potassium oxide (K2O), 0-1 wt % iron oxide (Fe2O3), 0-0.5 wt % titanium oxide (TiO2), and 0-0.5 wt % sulfur trioxide (SO3), plus a variety of other optional materials. The amorphous nature of the glass allows it to be reclaimed and reused during the manufacture of new soda-lime-silica glass articles. For example, recycled glass—typically referred to in the industry as “cullet”—of the soda-lime-silica variety may be combined with virgin raw materials (e.g., sand, soda ash, limestone, etc.) to provide a raw material feedstock that is fed to a furnace and melt processed in the conventional way.
Cullet is used in glass forming operations to reduce furnace energy consumption while also achieving other economic and process efficiencies. The inclusion of cullet in the raw material feedstock reduces furnace energy consumption since melting cullet requires less energy than melting virgin raw materials due to the fact that cullet is already chemically homogenized glass and, therefore, does not have to proceed through lengthy and complex endothermic glass forming reactions within the furnace. Mixing cullet into the raw material feedstock also reduces the amount of virgin raw materials that are consumed, which, in turn, reduces the amount intermediate corrosive oxides that are produced in the glass melt. The ability of cullet to decrease furnace energy consumption and contribute to a less corrosive glass melt can extend the service life of the furnace by, on average, up to 30%, while at the same time improving furnace output. Moreover, the addition of cullet to the raw material feedstock simplifies operation of the furnace by reducing the amount of CO2 and other gas emissions that need to be managed.
Cullet has traditionally been acquired from the contingent of waste glass produced at a glass manufacturing plant as well as from post-consumer glass recycling operations including municipal and/or commercial recycling facilities. While these sources of cullet can meet glass manufacturing consumption needs and schedules in most instances, the cullet acquired from those sources is subject to a certain level of variability, even within the same batch or lot, particularly with respect to the color of the glass. These types of sourced cullet may also include foreign residual impurities from contaminants such as paper, plastic, adhesives, ceramics, bottle caps, metals, dirt, and other impurities found in a post-consumer or industrial waste glass stream. The extent to which the acquired cullet has been cleaned of residual impurities and sorted according to color can affect how much of the cullet can be included in the raw material feedstock for a particular glass forming operation. For instance, in the production of flint glass articles, the cullet used is typically restricted to flint glass cullet only, while in the production of green or amber glass articles there is more leeway to mix cullet of different colors.
An attractive source of cullet that does not exhibit the variances inherent in standard post-consumer and industrial waste glass operations can be found in the solar energy industry. Specifically, thin film solar modules include glass substrates made of low iron soda-lime-silica glass that sandwich a thin film photovoltaic circuit having multiple thin film solar cells. A superstrate configured Cd/Te thin film solar module, for example, includes a front contact of one or more transparent conducting oxide (TCO) layers deposited onto an inner surface of a front glass substrate, a cadmium sulfide (CdS) n-type semiconductor layer and a cadmium telluride (CdTe) p-type semiconductor layer deposited in that order onto the front contact of TCO layer(s), and a back contact of one or more layers of electrically conductive material deposited onto the CdTe semiconductor layer. These layers are laser and/or mechanically scribed to a high precision during manufacturer of the module—often referred to as the P1, P2, and P3 scribs—to divide the module into monolithically integrated individual solar strip cells. The solar cells are then encapsulated by an adhesive layer, which is typically composed of ethyl vinyl acetate (EVA), to protect the cells and adhere the thin-film coated front glass substrate to a back glass substrate. Additionally, in order to facilitate the delivery of current to and from a given module via a junction box, one or more electrically conductive contact strips including interconnect bars and bus bars are disposed across the back contact of the various cells. The electrically conductive contact strips may be composed of a metal such as solder-coated copper. An example of a commonly-employed solder-coated copper is tin-coated copper.
The recovery of soda-lime-silica cullet from spent thin film solar modules is challenging since current recycling techniques are unable to cleanly isolate and separate the substrate glass from other materials of the module—most notably the adhesive layer and the electrically conductive contact strips. For instance, in current recycling processes for Cd/Te solar modules, the solar module or a portion of the solar module is jaw crushed and hammer milled into small pieces. The pieces of the front glass substrate include TCO and semiconductor material since there is strong adhesion between the front glass substrate and the TCO and semiconductor layers, but poor adhesion between the semiconductor layers and the adhesive layer, especially if the adhesive layer is EVA. The pieces of the of the back glass substrate, on the other hand, include adhesive layer material and possibly remnants of the electrically conductive contact strips that are embedded in and adhered to the adhesive layer. At present, there is no way to efficiently and adequately remove the contact strip remnants from the pieces of the back glass substrate. In that regard, if the electrically conductive contact strips are formed of solder-coated copper, the cullet obtained from the Cd/Te solar modules tends to have an unacceptably high copper content when melted, which is problematic since copper contamination can lead to commercial variations in subsequently manufactured glass articles.